Aircraft antenna balance techniques

ABSTRACT

Antenna balance and coupling circuitry for providing rejection of radio frequency signals produced by airframe currents flowing in a vertically polarized dipole antenna installed on an airborne structure.

United States Patent 91 Keenan et al.

[ Jan. 29, 1974 AIRCRAFT ANTENNA BALANCE TECHNIQUES Assignee:

Filed:

The United States of America as represented by the Secretary of the Army, Washington, DC.

Oct.

Appl. No.: 768,978

US. Cl 343/114, 343/705, 343/730, 343/820 Int. Cl. G01s 3/10 Field of Search.... 343/747, 705, 820, 730, 114

[56] References Cited UNITED STATES PATENTS 1,904,772 4/1933 Taylor 343/730 X 2,313,046 3/1943 Bruce 343/747 X Primary Examiner-Benjamin A. Borchelt Assistant Examiner-Richard E. Berger Attorney, Agent, or Firm-Edward J. Kelly; Herbert Berl; Milton W. Lee

[ ABSTRACT Antenna balance and coupling circuitry for providing rejection of radio frequency signals produced by airframe currents flowing in a vertically polarized dipole antenna installed on an airborne structure.

4 Claims, 2 Drawing Figures ATTENU- PHASE f" ATOR SHIFTER 59- 29. 51 DIPOLE MODE Y" OUTPUT 52/ 42 4| ATTENU- PHASE ATOR sun-"ran 30 3| COMMON MODE OUTPUT PAIENIEB 3.789.412

FIG. 1

ll UPPER I6 /MONOPOLE 0 MMON MODE I? MCURRENT LOAD l2 LOWER '6 r MONOPOLE DIPOLE MODE CURRENT FIG. 2

ATTENU- PHASE ATOR SHIFTER 10 39 E DIPOLE MODE OUTPUT ATTENU PHASE ATOR SHIFTER I2 30 J COMMON MODE OUTPUT INVENTORS,

MICHAEL G. KEENAN WILL/AM M. LYNCH a JOHN H. PRIEDIGKEIT jig L5 #WJQQ ATTORNEYS AIRCRAFT ANTENNA BALANCE TECHNIQUES BACKGROUND OF INVENTION This invention relates to an antenna arrangement utilized in high frequency radio direction finding systems and more particularly to a technique for balancing a di pole antenna mounted on an electrically conductive surface structure and for effecting the minimization of unwanted common mode signals.

High frequency radio direction finding systems are well established in the art. One of the most widely used systems is an airborne direction finding system comprising a pair of balanced vertical dipole antennae that provide a figure eight pattern with nulls fore and aft. Bearings are obtained by turning the aircraft for minimum received signal.

A null type homing system requires an antenna configuration that will produce a horizontal plane radiation pattern with a null on the forward axis of the aircraft. A pair of differentially connected vertical dipoles, will produce such a radiation pattern.

SUMMARY OF THE INVENTION The general purpose of this invention is to provide a vertical dipole antenna system which will provide an aural null or a left-right homing capability on the vertical polarization component of a target signal. Coupling circuitry for isolating a vertically polarized antenna from an airborne structure effectively minimizes any distortion in the antenna radiating pattern and reduces response of the vertically polarized antenna to horizontal or cross polarization.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a current flow diagram relative to an antenna mounted on a conducting surface.

FIG. 2 shows a block diagram of the delay circuit envisioned by the immediate invention.

DESCRIPTION OF THE INVENTION The circuitry envisioned by the drawings exemplifies a technique for balancing a dipole antenna when mounted on an electrically conducting surface structure, such as the wing of an aircraft, so as to minimize the signal coupled into the dipole antenna circuit due to radio frequency currents flowing on the conductive surface of the structure.

With reference to FIG. 1, it becomes readily apparent that the current in load 17 is the vector sum of a current 16 proportional to the desired vertical dipole current and a current 15 proportional to an undesired horizontal current. Depending upon the ratio of these two currents, the resultant antenna patterms may resemble those characteristic of a small, isolated, vertical dipole or those of a large, fat, horizontal dipole (wing mode of the aircraft). For frequencies well above the resonance of the horizontal dipole mode, the balance of the vertical dipole currents may be obtained with reasonable care. However, as the operating frequency approaches the resonant frequency of the horizontal dipole, the vertical dipole balance becomes very critical and frequency-sensitive, and unless great care is taken, the radiation patterns resemble those characteristic of a horizontal dipole rather than those of a vertical dipole.

FIG. 1 shows an electrically conducting surface 10, such as an aircraft wing, on which there has been placed a vertical dipole antenna. By virtue of the impedance existing between each half of the dipole (i.e., monopoles 11 and 12) and the conducting surface 10, each monopole may be considered as being a physical and electrical extension of the conducting surface. Thus, some of the currents that flow on this conducting surface 10 (common mode currents 15) when it is placed in an RF field leave this conducting surface and flow into the two monopoles 11 and 12 comprising the dipole antenna. When a load 17, such as a sensitive high frequency radio receiver, is connected to the terminals 13 and 14 of the dipole antenna, the currents flowing into the load 17 consist of the difference between the upper and lower monopole common mode currents flowing in monopoles 11 and 12 as well as the desired dipole mode current 16.

The level of the common mode currents may greatly exceed the level of the desired dipole mode currents, whereby the radiation pattern of the electrically unbalanced dipole antenna mounted on a conducting surface may be characteristic of the mounting surface rather than the dipole itself.

The balancing network of FIG. 2 alleviates this problem by utilizing RF signal attenuators 20 and 30, phase shifters 21 and 31 and coupling transformers 40 and 50, to permit the adjustment of the amplitude and the phase of the RF signals from the two monopole antennae 11 and 12. When these two signals are combined in the transformer 40 to form the desired dipole signal, there will be a maximum rejection of the unwanted common mode signal.

Two transformers 40 and S0 in a hybrid connection are shown to provide separate outputs for the dipole and the monopole modes. Although this transformer connection may not improve the rejection of the unwanted monopole mode, it provides a means for monitoring the unwanted common monopole mode. It also provides a means of comparing the levels of the two modes to determine the horizontal/vertical polarization ratio of the incident RF field.

In practice, the antenna system is illuminated with a linearly polarized RF field whose electrical field component is aligned with the principal dimension of the mounting surface. The attenuators 20 and 30 and phase shifters 21 and 31 are adjusted for a minimum dipole mode signal output. A plot may then bemade of the dipole mode output vs. polarization angle of the signal source to confirm that a minimum response is achieved when the electric field is parallel to the mounting surface.

The dipole balance technique of the above described invention very clearly renders possible the obtaining of antenna radiation patterns that resemble those of a dipole antenna in space when the antennas are mounted on structure whose dimensions approximate one-half wavelength at the signal frequency. This technique is not limited to the installation of antenna on aircraft. It has been demonstrated that it applies to antennas mounted on metallic booms and towers as well as airborne structures.

It should be understood, of course, that the foregoing disclosure relates to only a preferred embodiment of the invention and that numerous modifications or alterations may be made therein without departing from the 3 4 spirit and the scope of the invention as set forth in the circuit to provide the desired dipole signal which appended claims. has been phase balanced and RF attenuated in the We claim: balancing networks for maximum rejection of the I. In a high frequency radio direction finding system, common mode signal. coupling circuitry for reducing the response ol'a vcrti- 5 2. The coupling circuitry as set forth in claim 1, cally polarized antenna arrangement to horizontal and wherein said output means includes a pair of transformcross polarization, comprising: ers connected in a hybrid arrangement whereby the dia pair of monopole antennae, constituting a dipole pole mode signal and the common mode signal repreantenna, vertically mounted on an electrically consent separate outputs. ductive surface; 3. The coupling circuitry as set forth in claim 1 a balancing network connected to each monopole wherein each said balancing network includes at least antenna; an attenuator and a phase shifter for providing an adoutput means for providing both a dipole mode outjustment of the amplitude and phase of RF signals reput signal and a common mode output signal; ceived from the two monopole antennae. each of said balancing networks electrically coupled 4. The coupling circuitry as set forth in claim 2 to said output means and to one of said monopole wherein each said balancing network includes at least antennae, whereby the received signal from each an attenuator and a phase shifter for providing an admonopole antenna and the unwanted common justment of the amplitude and phase of RF signals remode signals emanating from the electrically con ceived from the two monopole antennae.

ductive body structure are combined in the output 

1. In a high frequency radio direction finding system, coupling circuitry for reducing the response of a vertically polarized antenna arrangement to horizontal and cross polarization, comprising: a pair of monopole antennae, constituting a dipole antenna, vertically mounted on an electrically conductive surface; a balancing network connected to each monopole antenna; output means for providing both a dipole mode output signal and a common mode output signal; each of said balancing networks electrically coupled to said output means and to one of said monopole antennae, whereby the received signal from each monopole antenna and the unwanted common mode signals emanating from the electrically conductive body structure are comBined in the output circuit to provide the desired dipole signal which has been phase balanced and RF attenuated in the balancing networks for maximum rejection of the common mode signal.
 2. The coupling circuitry as set forth in claim 1, wherein said output means includes a pair of transformers connected in a hybrid arrangement whereby the dipole mode signal and the common mode signal represent separate outputs.
 3. The coupling circuitry as set forth in claim 1 wherein each said balancing network includes at least an attenuator and a phase shifter for providing an adjustment of the amplitude and phase of RF signals received from the two monopole antennae.
 4. The coupling circuitry as set forth in claim 2 wherein each said balancing network includes at least an attenuator and a phase shifter for providing an adjustment of the amplitude and phase of RF signals received from the two monopole antennae. 